1. Field of Invention
The present invention relates to aliphatic polyesters and processes for manufacturing the same. More particularly, the present invention relates to modified polyesters and processes for manufacturing the same.
2. Description of Related Art
Aliphatic polyesters are known to be biodegradable and they have significant potential in various fields, such as molded plastic, nonwoven webs, and tissue engineering. However, most known condensation-polymerized aliphatic polyesters do not have desirable thermal and mechanical properties for practical applications since there are no benzene rings present in their backbones. Furthermore, it is very difficult to increase the molecular weight of the aliphatic polyester over 15,000 due to its poor heat stability. As such, the use of these aliphatic polyesters is limited.
Various prior efforts have been made to address this problem.
For example, introduction of amide bonds into the polyester chains can enhance the hydrogen bonding between the aliphatic polymer chains, thereby producing polyester amides with improved flexibility. To this end, the amount of amides present in the conventional polyester amides is generally about 40-70 mol %. However, it is well known that the hydrolysis of amide bonds of the polyamides is quite slow (as compared with the ester bonds of the aliphatic polyesters), and thus, the biodegradability of such polyester amides would be compromised.
Another possible way of improving the mechanical properties of the aliphatic polyesters involves in the addition of branching agents having three functional groups.
For example, U.S. Pat. No. 5,436,056 disclosed the production of high-molecular weight aliphatic polyesters with a tri-functional or tetra-functional ingredient. The branching agents are capable of cross-linking the molecular chains of the aliphatic polyesters thereby increasing the molecular weight, and hence, the mechanical strength, of the resultant polyesters. In this case, the cross-linking level of the reaction should be carefully controlled in order to obtain aliphatic polymers with desirable mechanical properties. This is because that a polymer without sufficient cross-linking level may be too brittle, whereas a polymer with excessive cross-linking level may become thermosetting, and in both cases, it is difficult to mold the polymer. In order to provide suitable level of cross-linking, the polymerization is usually carried out in a moderate way where the reaction temperature is relatively low. However, the manufacture of such polymer under such condition is extremely time-consuming and cost-intensive, thereby hindering the possibility of its mass production.
In view of the foregoing, there exists a need in the art for providing an inexpensive and easy-to-manufacture modified polyester with appropriate mechanical properties, as well as a corresponding process for manufacturing it in a time- and cost-effective way.